The present invention relates to a frequency synthesizer which is used for example, for varying a received signal frequency, in a spectrum analyzer, or for supplying signals of various frequencies to a device under test, that is, used as the so-called signal generator for generating signals of various frequencies. In particular, the invention pertains to a frequency synthesizer which permits frequency variations over a wide frequency band at short intervals.
In a conventional frequency synthesizer, the output from a voltage controlled variable frequency oscillator is frequency divided by a variable frequency divider, the frequency-divided output and a reference signal are phase compared and, by the phase-compared output, the oscillation frequency of the variable frequency oscillator is controlled, bringing the output frequency of the variable frequency divider into agreement with the frequency of the reference signal. That is, the oscillation frequency of the variable frequency oscillator is altered by changing the frequency dividing ratio of the variable frequency divider. In the prior art frequency synthesizer, it is difficult, in general, to select the frequency of the reference signal low, so that it is impossible to vary the output frequency by steps of 1 Hz, for instance. Furthermore, when the frequency step is selected small, the frequency dividing factor of the variable frequency divider must be augmented markedly, which leads to a decrease in the loop gain, resulting in increased settling time in which to reach a set frequency. Besides, the compared output from the phase comparator is applied to a low-pass filter which is required to have a cutoff frequency lower than the frequency variation interval, but such a low-pass filter is difficult to produce. Moreover, for effecting frequency sweep about the set frequency, the phase-compared output is sample-held and the sample-held output and a sweep signal are superimposed on each other and then the superimposed signal is applied as a control signal to the variable frequency oscillator. In this case, as a frequency control loop is cut off, the frequency stability becomes low.
Furthermore, in the prior art, the output from a variable frequency crystal oscillator is frequency multiplied by a frequency multiplier; the multiplied output is phase compared by a phase comparator with the oscillation output from a voltage controlled variable frequency oscillator and, by the phase-compared output, the oscillation frequency of the voltage controlled variable frequency oscillator is controlled, thereby to cause the frequencies of before the inputs to the phase comparator to coincide with each other. Namely, the oscillation frequency of the voltage controlled variable frequency oscillator is changed by controlling the oscillation frequency of the crystal oscillator. In this case, even if a control signal for the crystal oscillator is set in a digital form, the oscillation frequency of the crystal oscillator does not always become as desired. Therefore, it is also necessary to prepare means for measuring the oscillation frequency of the crystal oscillator.